An unshielded lamp in the direct line of vision is a potential source of direct glare caused by excessive luminance. There are two ways of avoiding direct glare: either the lamp is shielded, e.g. by the housing itself or by fitting solid vanes or louvres, or the high level of luminance is reduced through the use of opal or prismatic diffusers.



The luminance limit angle a of a luminaire as measured from the horizontal plane is the maximum angle at which the lamp is not yet in the direct line of vision.

In the simplest case reflector luminaires are shielded by vanes made of metal or plastic placed parallel and at right angles to the axis of the lamp. The luminance limit angle is dependent on the combination of vane height h and vane spacing a.


In EN 12464, Part 1 minimum luminance limit angle amin is specified per lamp type and glare cut-off class :

 

Range of mean luminance cm/m2

Direct glare cut-off class

   

A

1

2

3

Fluorescent lamps

L = 2·104

20°

10°

Compact fluorescent lamps

2·104 < L = 4·104

20°

15°

High pressure discharge lamps with fluorescent or matt bulbs, low-pressure sodium lamps

4·104 < L = 50·104

30°

20°

10°

High pressure discharge lamps with clear bulbs or tubular types, incandescent lamps with clear bulbs

L = 50·104

30°

30°

15°

10°